Conjugation of water soluble polymers to bioactive agents and colloidal carrier systems such as nanoparticles, micelles, niosomes and liposomes, is used to prolong their circulation half-life and reduced toxicity in human body, which in turn provides superior efficacy, less dosing frequency and better patient compliance. The superior efficacy of polymer conjugated bioactive agents is demonstrated by several marketed products like PEG-Intron®, Neulasta®, Somavert®, Oncospar®, Adagen® and PEGASYS®, wherein polyethylene glycol, hereafter referred to as ‘PEG’ is conjugated to various protein therapeutics. These products exemplify the use of water soluble polymer conjugated to water soluble therapeutic proteins to prolong the circulation half life.
Conjugation of water soluble polymers to water insoluble bioactive agents, is being investigated to enhance the solubility of the water insoluble active. Examples of such systems under development are polyethylene glycol conjugated camptothecin, polyglutamic acid conjugated paclitaxel and polyhydroxymethacrylamide conjugated paclitaxel.
Water soluble polymers may be coupled to hydrophobic polymers in block architecture. These block copolymers spontaneously self assemble in aqueous medium to form polymeric micelles and nanoparticles. These polymeric micelles and nanoparticles, have a hydrophobic core and an outer hydrophilic shell around it. The hydrophobic inner core can incorporate water insoluble hydrophobic drugs by hydrophobic association. Therefore these polymeric micelles and nanoparticles can be used for drug delivery.
An important characteristics of polymer conjugated systems is its passive accumulation at tumour site by size effect, known as epr (enhanced permeability and retention) effect’ due to the leaky nature of the tumor vasculature. This passive targeting is the mechanism of action of an anti-tumor agent, SMANCS, approved in Japan for liver cirrhosis. SMANCS consists of low molecular weight styrene maleic anhydride copolymer conjugated to neocarzinostatin through the anhydride groups present in the polymer. Although the molecular weight of SMANCS is about 16–17 kDa, it forms larger aggregates with serum albumin. The aggregated size of the conjugate, 80 kDa, is said to responsible for the spontaneous but passive accumulation of SMANCS at the tumor site.
The above described passive targeting mechanism is also demonstrated by nanoparticles and polymeric micelles having diameter less than 200 nm, provided they have a prolonged plasma circulation half life. Since the nanoparticles and polymeric micelles are inherently coated with a hydrophilic water soluble polymer, they are expected to enhance plasma circulation half life and hence passive accumulation at the tumor site. Unlike nanoparticles and polymeric micelles, liposomes do not inherently have a hydrophilic polymer coating to prevent uptake by Mononuclear Phagocyte System (MPS) and rapidly cleared from the circulation to organs rich in phagocytic cells, like liver, spleen and bone marrow.
Liposomes are small vesicles having one or more concentric lipid bilayers enclosing an aqueous space. Because of their structural versatility in terms of size, surface charge, lipid composition, bilayer fluidity and because of their ability to encapsulate almost every drug, their importance as drug delivery systems was readily appreciated. However, on intravenous injecting of liposomes, these are recognized as foreign particles by the Mononuclear Phagocyte System (MPS) and rapidly cleared from the circulation to organs rich in phagocytic cells, like liver, spleen and bone marrow.
Several possibilities to reduce this effect have been identified, such as decreasing the particle size of the liposomes and changing the surface charge of the liposomes. Another development relates to surface modification of the liposomes by the introduction of specific hydrophilic polymeric components on the liposomal surface, which groups reduce protein adsorption on the particle surface. Consequently such liposomes are protected against recognition by cells of the MPS and have a prolonged residence time in the general circulation. A well-known example of modification of the liposomal surface is the incorporation during the preparation of liposomal compositions of a lipid derivative of the hydrophilic polymer polyethylene glycol (PEG). Usually this hydrophilic polymer is terminus-modified with a hydrophobic moiety, which is the residue of a phosphatidyl ethanolamine derivative or a long-chain fatty acid. Polyethylene glycol per se is a rather stable polymer which is a repellent of protein adhesion and which is not subject to enzymatic or hydrolytic degradation under physiological conditions.
Good results with respect to extending plasma half life and diminishing accumulation into the organs rich in phagocytic cells have been obtained following intravenous administration of liposomes, having a PEG-grafted surface, to various animal species and also to human beings (Storm G., Belliot S. O., Daemen T. and Lasic. D. D.: Surface modification of nanoparticles to oppose uptake by the mononuclear phagocyte system in Adv. Drug Delivery Rev. 17, 31–48, (1995); Moghimi S. M., Hunter A. C. and Murray J. C.: Long-circulating and target-specific nanoparticles; theory to practice in Pharmacol. Rev. 53, 283–318, (2001)). Marketing approvals for such liposomal preparations, containing doxorubicin, have been obtained.
Until now the commercially available preparations based on PEG-liposomes are aqueous suspension preparations. It is well-known that the shelf life of liposomal aqueous suspension preparations in general and also of PEG-liposomes is rather limited. Several techniques how to remove the vehicle or continuous phase of such preparations are known, such as, spray-drying, diafiltration, rotational evaporation, and freeze-drying. Recently a freeze-drying method, which improved the long term shelf life of PEG-liposomes, containing the technetium-chelator hydrazino nicotinamide, was proposed (Layerman P., van Bloois L., Boerman O. C., Oyen W. J. G., Corstens F. H. M. and Storm G.: Lyophilisation of Tc-99m-HYNIC labelled PEG-liposomes in J. Liposome Res. 10(2&3), page 117–129 (2000)), but further investigations into the results and applicability of this technique to liposomal preparations are required.
The disadvantages inherent to the use of polyethylene glycol urged investigators to look for alternative polymers. Many polymers have been suggested as suitable candidates for derivatizing them with (vesicle-forming) lipids for incorporation into liposomes (see e.g. EP-0688207). The hydrophilic water soluble polymers poly(vinylpyrrolidone), poly(acryloylmorpholine), poly(2-(m)ethyl-2-oxazoline, polyacrylamide and polyglycerol have shown to prolong the circulation time of liposomes after intravenous administration to a certain extent. However, until now such lipid polymer conjugates have not been applied in commercially available drug preparations, mainly because they have not shown any advantages over the known lipid-PEG-conjugates.
Therefore there still is a need to find a polymer which can be derivatized with a lipid to enable incorporation into colloidal carrier compositions, such as liposomes, such polymer having long-circulating properties.